The present invention relates to a gas cooled nuclear reactor with a cylindrical core of a pebble bed of spherical elements (e.g., fuel elements, absorber elements graphite elements),surrounded on all sides by a graphite reflector consisting of top, side and bottom reflectors, and with both trim and shutdown means, consisting of a plurality of absorber rods displaceable in vertical channels of the side reflector.
A nuclear reactor of this type is described in German Application P 35 18 968.1 which corresponds to allowed U.S. application Ser. No. 06/866,921. It is characterized by a compact configuration and simple construction, whereby low energy costs may be obtained. The stationary pile of operating elements enables power operation to be achieved for approximately 10 to 40 years. The operating elements and the absorber rods are then replaced. The nuclear reactor, having a capacity of approximately 10 to 20 MWe, preferably serves as a heating reactor.
For a safe start-up of nuclear reactors, neutron fluxes must be measured with the core in the shutdown state. This requires a neutron source and start-up flux measuring instruments. The neutron source must satisfy two conditions:
the neutron emission of the source (source strength) must be high enough so that 5 pulses per second are registered in the neutron flux detectors of the start-up measuring column; PA1 the distance of the source from the detector must be large enough so that with a multiplication factor of the reactor of 0.99, a maximum of 5% of the neutrons detected in the start-up measuring column come directly from the source.
The first condition guarantees a measuring signal that is adequately statistically accurate, while the second assures that the measuring signal is determined essentially by the core and not by the source.
In the gas cooled nuclear reactor built heretofore with a pebble bed of spherical operating elements, e.g., the THTR-300 MWe (a so-called pellet pile bed reactor), the neutron sources are displaceably positioned in bores of the side reflector. DE-OS 30 47 098 discloses, for example, a transport device for the introduction of a start-up neutron source into the side reflector of a high temperature reactor. This peripheral location of the neutron source in the zone close to the side reflector is neutron-physically disadvantageous and requires high source strengths if the two aforementioned conditions are to be satisfied.
The most favorable location of a neutron source is in the center of the core. However, this optimum position cannot be realized in the THTR-300 MWe and other planned nuclear reactors of this type, as the operating elements which pass through the core once or several times, are circulated in operation. Only during the initial loading of the core, when the operating elements are not yet circulated, may neutron sources be installed temporarily in the core itself. The neutron sources are housed in this case in a rod, which--in a manner similar to the absorber rods for the shutdown of the nuclear reactor--are inserted directly into the pebble bed.
In the case of small bed reactors with a stationary core, in which no absorber rods directly insertable into the pile are provided, the location of neutron sources in bores of the side reflector poses particular problems, as the control and shutdown of these reactors is effected by means of absorber rods displaceable in the side reactor, the so-called reflector rods, and a sufficient number of positions must be available for the reflector rods. The highest possible effectiveness is required of the reflector rods of such a small reactor, in order to be able to bind the excess reactivity necessary for the longest possible service life of the reactor. Long service lives without any addition of fuel make it necessary to locate the highest possible number of reflector rods on the periphery of the reactor core. There is, therefore, no room for neutron sources in the area of the side reflector close to the core.
It is the object of the present invention to locate the neutron sources in a nuclear reactor of the aforementioned type in a manner such that no positions needed for reflector rods are occupied and that simultaneously an effective neutron physical effect is achieved.